| The spherical fuel element that contains tens of thousands Tristructual ISOtropic(TRISO)particles has favorable features on high-temperature performance,inherent safety,nuclear non-proliferation,etc.Therefore,it attracts considerable attentions from both academics and industries and be employed in several conceptual reactor design.The key feature of these conceptual reactors is the core consisted of pebble fuels randomly packed in reactor vessel.This kind of reactor form is named as pebble bed,or more generally,packed beds(a special case with uniform spherical particles)from the thermal-hydraulics’ research perspective.The packed bed can be divided into two categories according to the vessel-to-pebble diameter ratio: conventional packed beds with high D/d ratios(generally D/d > 10)and packed bed with low D/d ratios(D/d ≤ 10).Recent studies have shown that the packed beds with low D/d ratios have advantages on pressure drop reduction and heat transfer enhacnemnt;on the other hand,their unique charateristics make the general research results based on packed beds with high D/d ratios inapplicable down to this range.To address the problem,this study concertrates on the characterization of packing structure and thermal-hydraulics of packed beds with low low D/d ratios.The major findings are as below:(1)The study on packing structure of packed beds includes two scopes: mean porosity variations and radial-axial porosity distributions.The mean porosity variations of packed beds with different D/d ratios were studied by experiments at the range of 1.2 ≤ D/d ≤ 7.1.The experimental data have certain fluctuations as increasing the D/d rations.To reach a theorical explanation on the fluctuations,a model—the layer-stacking model—was proposed to depict the packing state of packed beds.Then,it was used to develop analytical expressions for mean porosity variations of packed beds at D/d < 3 and to give physical sights into the mean porosity variations as varying D/d ratios.As a result,the fluctuations are confirmed a reflection of the discreteness and non-monotonicity nature of the mean porosity variations.The radial-axial porosity distribution of packed beds was investigated through numerical simulations.The comparison between the simulation results and the predictions by classical correlations,namely Mueller correlation and De Klerk correlation,shows enlarging deviations as decreasing the D/d ratios,resulting from the randomness-ordering packing structure transformation.Accordingly,the packed bed with low D/d ratios can be classified into two categories: the quasi-structured packed bed(D/d ≤ 4)and random packed beds(D/d > 4).(2)The random packed beds were investigated on pressure drop and particle-to-fluid convective heat transfer performance.It was found that some classical empirical correlations based on packed beds with high D/d ratios(such as Carman correlation for pressure drop and Wakao correlation for convective heat transfer)give a good agreement with the simulation results.An unexpected result is that,although some recently developed pressure drop correlations were design to improve the prediction accuracy for the packed beds with low D/d ratios,they show an inferior performance in this comparison.Two major reasons may contribute to this result: first,few previous studies realized the randomness-ordering packing structure transformation and treated the two kinds of packed beds as the same,which cause the resulted empirical correlations to give an improved prediction for neither the random nor the quasi-structured one;second,the corrected correlations based on the Ergun type fail to characterize the packed beds pressure drop at high Reynolds numbers.Then,the study was extended to local fluid flow and heat transfer variations in the beds.A typical ununiform fluid flow velocity-temperature distribution,namely the wall channeling effect,was observed in every random packed bed,which is consistent with the general research opinion from the literature.As for the particleaveraged convective heat transfer capacity,a clear increasing trend was identified at a distance approximately3 d in the entrance region and no difference occurred among the particles from different radial positions.(3)The packed beds at 2 ≤ D/d ≤ 3 were considered as a representative of the quasi-structed packed bed to evaluate the influence of the packing structure transformation on the thermal-hydraulics.The packed bed at this D/d range has a special configuration and it is therefore named as Hollow Structured Packed Bed(HSPB).The HSPBs present completely different characteristics than the random on thermal-hydraulic performance.For the global behaviors,both pressure drop and convective heat transfer capacity of HSPBs showed a decreasing trend as smaller particles were packed into a given column(in other words,decreasing D/d ratios)and the empirical correlations that gave a good prediction to the random beds fails to track these variations;for the local behaviors,the fluid flow showed significant channeling effect along the axis instead of that at the wall.Finally,an overall heat transfer evaluation was performed among the random,quasistructured,structured packed beds.The quasi-structured packed beds show superior performance in this comparison.(4)A general strategy was proposed for packed beds’ thermal-hydraulics optimization—the ordered packing reinforcement,inspired by the superior performance of structured as well as quasi-structured packed beds than the random.Since the confining wall is found to benefit the packings’ ordering,additional walls are introduced into packed beds,forming a radial layered configuration.At the same time,the wall is designed with porous material to allow free flow mixing.The CFD results showed that the radial layered confguration is capable of achieving an optimized fluid flow distribution as well as improved overall heat transfer performance. |
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